1. Field of the Invention
This disclosure relates generally to load balancing among servers. More particularly but not exclusively, the present disclosure relates to achieving load balancing by, in response to resolving a DNS query by a client, providing the address of a server that is expected to serve the client with a high performance in a given application.
2. Description of the Related Art
Under the TCP/IP protocol, when a client provides a symbolic name (“URL”) to request access to an application program or another type of resource, the host name portion of the URL needs to be resolved into an IP address of a server for that application program or resource. For example, the URL (e.g., http://www.foundrynet.com/index.htm) includes a host name portion www.foundrynet.com that needs to be resolved into an IP address. The host name portion is first provided by the client to a local name resolver, which then queries a local DNS server to obtain a corresponding IP address. If a corresponding IP address is not locally cached at the time of the query, or if the “time-to-live” (TTL) of a corresponding IP address cached locally has expired, the DNS server then acts as a resolver and dispatches a recursive query to another DNS server. This process is repeated until an authoritative DNS server for the domain (e.g., foundrynet.com, in this example) is reached. The authoritative DNS server returns one or more IP addresses, each corresponding to an address at which a server hosting the application (“host server”) under the host name can be reached. These IP addresses are propagated back via the local DNS server to the original resolver. The application at the client then uses one of the IP addresses to establish a TCP connection with the corresponding host server. Each DNS server caches the list of IP addresses received from the authoritative DNS server for responding to future queries regarding the same host name, until the TTL of the IP addresses expires.
To provide some load sharing among the host servers, many authoritative DNS servers use a simple round-robin algorithm to rotate the IP addresses in a list of responsive IP addresses, so as to distribute equally the requests for access among the host servers.
The conventional method described above for resolving a host name to its IP addresses has several shortcomings. First, the authoritative DNS server does not detect a server that is down. Consequently, the authoritative DNS server continues to return a disabled host server's IP address until an external agent updates the authoritative DNS server's resource records. Second, when providing its list of IP addresses, the authoritative DNS server does not take into consideration the host servers' locations relative to the client. The geographical distance between the server and a client is a factor affecting the response time for the client's access to the host server. For example, traffic conditions being equal, a client from Japan could receive better response time from a host server in Japan than from a host server in New York. Further, the conventional DNS algorithm allows invalid IP addresses (e.g., that corresponding to a downed server) to persist in a local DNS server until the TTL for the invalid IP address expires.